Magnetic surveying apparatus



Nov. 23, 1943. v E. P. HARRISON MAGNETIC SURVEYING APPARATUS a Sheets-Sheet 1 Filed July 23, 1940 Q \\wlI Nov. 23, 1943. E. P. HARRISON MAGNETIC SURVEY ING APPARATUS Filed July 23, 1940 s Sheets-Sheet 2 EQEQBQ N06. 23, 1943. P, HARRISON 2,335,117

mummc sunvmm APPARATUS Filed July 23,1940 3 Sheets-Sheet 3 Patented Nov. 23, 1943 UNITED STATES" PATE NT orrics MAGNETIC SURVEYING APPARATUS Edward Philip Harrison, Barkingside,

England,

assignor ot one-third to Arthur Joseph Hughes,

Ohigweil Row, Henry Hughes land, a British Essex, England, and one-third to & Son Limited, London, limited-liability company E r-H Application July 23, 1940, Serial No. 348,975 In Great Britain July 25, 1939 4 Claims. (01175-183) l invention relates to improved magnetic surveying apparatus for use on or near to the surface oi the earth and has for its-object to constitute an instrument which can be used more expeditiously in the field, which is of a robust character suitable for use in the field, which is devoid of delicately poised moving parts and which can be produced at less cost than is p08! sible with known devices for. a similar p se. It isknown that the impedance of a suitably annealed wire composed of a high permeability nickel-iron alloy properly supported and carrying alternating electrical current decreases when the axial component of the external magnetic field increases. It i this property or high permeability substances' 'such as a nickel-iron alloy, that I employ in my improved magnetic surveying apparatus.

Figure 4 is a diagrammatic showing of the instrument arrangements in the development according to Figure 3, and

If two such wires or high permeability'nlckeliron alloy, known commercially as Mumetal, are separately mounted and kept near together and located vertically or horizontally and placed in adiacent'arms of a bridge, the latter may be balanced. When, however, one 01 the wires of high permeability nickel-iron alloy is removed to a position some distance from the first wire, the bridge balance will be upset provided the vertical held at a distance is diilerent from that in location of balance. The amount oi "out of balance as shown in the indicating instrument is a measure of the diflerence in magnetic field value between the two positions.

The invention consists of an improved magnetic surveying apparatus in which the change in impedance or a wire 01' high permeability nickeliron alloy is employed to detect a change in the external magnetic field and is characterised in that two such similar wires are arranged as two arms in a balanced alternating current bridge network and that means are provided for determining the correction necessary to restore the balance when the impedance of one 01' the said wires changes due to a change in the external magnetic field.

In order that the nature of the invention may 4 be the better understood, examples thereof will now be described in relation to the accompanying drawings, reference being had to the several figures thereof and to the letters of reference Figure 5 is an electrical diagram 01' a conveniently arranged installation applicable to Figure 4, but omitting the localpolarising circuits to avoid confusion; T

' 'I'hebasis oi'the instrument shown in Figure l is an alternating current Wheatstone bridge network or which a and b are two similar wires or an appropriate alloy possessing the property rei'erred to above, 0 is a variable-inductance in series with a variable non-inductiv resistance d;

' e is an inductivelyarranged fixed resistance havconvenient results have been positively anchored to the ing an impedance temperature coeillcient similar to that or a or b and an impedance substantially the same as that oia or b or c and d, f is a source of alternating current and is a current indicating meter. V

The wires a and buseiully may be about nine inches long and .0175 inch in diameter, correspending to No. 26 S. W. (3., and may be composed of an alloy comprising about 16.6% of iron and about 77.3% of nickel, together with say 5% of copper and small percentages of manganese, chromium, silicon and/or other elements to give the high permeability characteristic. It is essential that such wire should be supported without material strain and' in the present instance obtained when the wire a (or b). is enclosed-(see Figure 2) in a tube a or electrical insulating material (such as of paper impregnated with an artificial resin) being metal terminal a at the upper end, whilst its lower end is connected to the spiralspring b positively anchored to the metal terminal b". The interior or the tube a has a filling a of viscous material, for example a paraflin such as petroleum Jelly, and one or more solenoids are wound upon the exterior or a for to vpolarising and/or compensating purposes. When a is equal to measuring the vertical component of the earths magnetic field the wires a and b have to be arranged vertically and for this purpose the terminal a includes a suspension 11 which may be an articulated universal joint, a ball and socket, a loop and hook (with or without knife edges) or a metal filament, undue oscillation of the suspended element being damped by vanes b mounted on the lower terminal b", immersed in a liquid b (such as oil or other liquid of a suitable viscosity) contained in a non-spill vessel b In order that the fixed resistance e shall have the same impedance temperature coefllcient as that of the high permeability nickel iron alloy of which the wires a and b are composed, the said resistance consists of two wires e and e which are electrically connected in the circuit in series or parallel and are composed of non-ferromagnetic materials which comply with the following considerations.

It is assumed that at temperature 0 the resistance of e is equal to that of e and the resistance temperature coefficients are '7 and e respectively. It can be shown that if the temperature is raised, the resistance temperature coefficient a of the combination is given by:

or more approximately if squares and products of coefficients are neglected. The resistance temperature coemcient of :a suitable high permeability nickel iron alloy may be considered to be .0024. The coefficient for platinum is .0038 and a copper zinc alloy such as brass, the resistance temperature coefficient ofv which is .00l,,can be selected so that if the wire e is of platinum and the wire. e is of the selected brass and the wires are of equal electrical ture coefiicient or aluminum and brass for example, which in this instance satisfy the condition:

is approximately equal to .0024, so that in this approximation y-i-e can be regarded as equal to .0048.

When summarised, the conditions to be fulfilled are: (I) The sum of the resistance temperature coefiicients of the materials used for the wires e ande in parallel must be equal to .0048. ,(II) The wires e and e must be of the same length and have the same resistance at some temperature 0, which for example may be the average day temperature. (III) The total resistance of the parallel combination should suit the resistance of each of the wires a or b of high permeability nickel-iron alloyapproximately. (IV) The inductance of the combination (usually when wound on acylindrical former) must be approxi- -mately equal to the inductance of each of the wires a or b.

i =l 2 a- R 01 where p =the specific resistance in microhms per square centimetre. l=the length of the wire in centimetres. d=the diameter of the wire in centimetres. a=the area of the wire in square centimetres. R=the resistance of the wire in ohms.

= .036 centrirnetre so the conditions are satisfied with stock commodities if three hundred and fifteen centimeters of 34 S. W. G. aluminum wire is arranged in electrical parallel with an equal length of 28 S. W. G. brass wire.

In practice the brass and aluminum wires are insulated from one another and connected at their ends by solderingor welding to form a par! allel combination which can be wound closely on a cylindrical bobbin having a groove say fivesixteenths of an inch wide and a core diameter of say one inch.

As the bridge network is symmetrical, any effects due to change of frequency and/or change of amplitude of the alternating current from the source I are negligible and, provided that the impedance temperature coeflicient of e is the same as that of a and/or b, effects due to diiierences of temperature between a and b may be ignored.

From the above it will be seen that the Wheatstone bridge network described above can be divided so as to constitute two instruments A and B as shown in Figure 3, connected only by a three core cable consisting of the midwire z and the conductors a: and y conveying electrical current to the distant instrument B.

If it is assumed that the magnetic field is constant throughout the region of the instrument A, the condition for no current in 9 (so far as the resistance component of the impedance is concerned) is:

the ohmic resistance of a the ohmic resistance of b and this relation continues to hold for so long as the instrument B is close to instrument A.

When, however, the instrument B is removed to a position distant to the instrument A, the balance of the bridge will be upset if the earth's field is different in the two positions and the value of the impedance correction will be a measure of the field intensity difference.

,Since the change of effective resistance of a wire of high permeability nickel iron alloy, placed in zero or Very small fields up to (say) 0.1 gauss, is very small (the resistancerfield curve being nearly parallel to the field axis throughout this region) in order to achieve the necessary sensitivity in a survey instrument when dealing with any components of the field near to zero, it is necessary to polarise the element so as to utilise a portion of the field resistance curve whose slope is greater than it isvery near to zero field. To

' carry this into effect a convenient method is to wind 9. solenoidal coil over the element and its mounting, and pass a suitable current through this coil thus applying to the wire an artificial 7 wires of high permeability nickel-iron alloy in any position whether vertical or horizontal and forms a very convenient method of reducing to zero any out-of-balance indications on the meter, thus throwing back the determination of a magnetic field or change of field on to a determination of electric current and the constants of a solenoid.

In the practical embodiment of the instruments A and B as shown in Figure 4, they are enclosed in non magnetic cases A and B respectively, the tube (1 containing the wire a has a winding a connected with a battery (1 through a switch a and the tube a containing the wire I) has a winding I) connected with a battery b through a switch b, so that the wire a and b can be polarised. The winding b is overwound with a compensating coil b having a, milliammeter b and a variable resistance 1) in series circuit with a direct current source 11 The instruments A and B as a whole are electrically energized by a unit comprising the oscillator C, filter C and amplifier rectifier C (described in relation to Figure 5) associated with a battery unit D.

The instruments A and B are used as follows: assume A is in position I and th vertical component of the earths magnetic field is. to be measured at position 2.

The instruments A and B are placed close together at position I, a and b are polarised, c and d are adjusted to balance the bridge network, B is moved to position 2 and b is adjusted to balance the bridge again. The reading of the milliammeter b will be a measure of the change of magnetic field and can be calibrated to read in magnetic units.

A convenient arrangement for generating the necessary alternating electrical current is shown in Figure 5 in which A and B are the surveying instruments side by side. C is a thermionic oscillator producing an alternating current at, for example, about 500 alternations per second. The output from C is applied to A and B through the transformer h. The out-of-balance current from A and B is passed through the two-stage constant-K-type low pass filter C cutting off at about 800 cycles per second to prevent any of the harmonics generated in a and b passing to the amplifier C The amplifier C is a resistance capacity coupled amplifier with two high frequency pentodes and a power pentode output transformer coupled to the circuit a or the circuit is by means of the switch I.

The circuit 7' includes a current meter 9' and an associated rectifier bridge so as to enable approximate balance-of the main bridge to be ef fected, whilst the circuit It includes a centre zero micro-ammeter k rectifiers k polarised by the outputs from the transformer h and a rheostat k having a variable tapping.

If initially the circuit is has been adjusted by meability conductors.

k sov that It indicates zero when the input from the main bridge is nil, any decrease in the impedance in one arm of the main bridge will produce,a deflection to one side of zero and any change in the other arm will produce a defiection to the other side, the value of th deflections positive or negative being proportional to the change of impedance in the main bridge arms and being a function of the change which has occurred in the magnetic field when B was moved away from A.

When instruments of the kind referred to above are to be used to measure a horizontal component of the earth's field, the wires a and b are arranged horizontally and the case A and/or B are provided with turntable mountings so that adjustment in azimuth can be eifected easily.

In some cases the wires a and b and the solenoids a and b b respectively are arranged in nonmagnetic clamps pivotally mounted on a support with a suspension (1 and vanes b by means of a counterpoised indexing contrivance which ensures the location of the wire of high permeability nickel-iron alloy either in a vertical or a. horizontal position.

A magnetic survey over a long lin may by one or other of these methods be carried out by a step by step process but the resistance of the connecting leads between A and B must not be too large comparedwith the resistance of one of the wires of high permeability nickel-iron alloy otherwise the sensitivity of the latter will be swamped by circuit conditions.

I claim:

1. A magnetic surveying apparatus comprising balancing means having two arms including similar conductors of high permeability, a third arm including an adjustable impedance and a fourth arm including a fixed nonferromagnetic resistance, said non-ferromagnetic resistance comprising two metal strips of substantially the same resistance at a given temperature and wound together inductively, the impedance tom-- perature coefiicient of the combined metal strips being substantially the same as the impedance temperature coefficient of each of said high per- 2. In a magnetic surveying apparatus provided with a substantially straight elongated conductor having a characteristic which changes due to changes in the external magnetic field, a support for said conductor including a point of support by which the conductor is adapted to assume consistent and accurate positions by action of gravity and means to dampen any tendency of the conductor to swing upon the one point support.

3. A magnetic surveying apparatus comp-rising balancing means divided into two relatively movable parts, each of said parts being provided with a similar conductor of high permeability connected in circuit with the balancing means, an

adjustable impedance associated with one of said conductors, a fixed non-ferromagnetic resistance ternal magnetic fields affecting said conductors I to suppress harmonics generated in said conductors, and means responsive to the out-of-balance current for indicating the difference in the external magnetic fields.

4. A magnetic survey apparatus, comprising an alternating current bridge network, having two arms includin similar conductors composed of an annealed'high permeability nickel-iron alloy, a third arm including an adjustable impedance and a fourth arm including a fixed non-ferro- -maznetlc resistance, said non-ferromagnetic resistance comprising two metal strips of substantially the same resistance at a given temperature and wound together inductivelmthe impedance temperature coemcient or the combined metal strips being substantially the same as the impedance temperature coemclent of each of said high 5 permeability conductors.

EDWARD PHILIP HARkISON. 

